1. Field of the Invention
The present invention relates to a solar cell module and, more particularly, to a solar cell module in which a light-incidence-side surface of a photovoltaic element is sealed by a covering member comprised of at least two layers including a sealant resin layer of a transparent, organic polymer resin and an outermost, transparent surface protecting film.
2. Related Background Art
In recent years, the increase in awareness of environmental issues is spreading on a worldwide scale. Among others, the concern over the warming phenomenon of the earth due to emission of CO.sub.2 is deepening, and demands are becoming greater for clean energy. Solar cells can be considered at present a promising clean energy source because of their safety and ease in handling.
Various forms of solar cells are known. Typical examples are as follows:
(1) Crystalline silicon solar cells PA1 (2) Polycrystalline silicon solar cells PA1 (3) Amorphous silicon solar cells PA1 (4) Copper indium selenide solar cells PA1 (5) Compound semiconductor solar cells
Among these, the thin film crystalline silicon solar cells, compound semiconductor solar cells, and amorphous silicon solar cells have recently been the subject of research and development on many fronts, because they can be made in a large area at relatively low cost.
Further, among these solar cells, the thin film solar cells, typified by an amorphous silicon solar cell in which silicon is deposited on a conductive metal substrate and a transparent, conductive layer is formed thereon, are considered to be promising for future module forms, because they are lightweight, impact-resistant, and very flexible. It is, however, necessary to cover the light-incidence-side surface with a transparent covering member to protect the solar cell, different from those obtained by depositing silicon on a glass substrate.
The most popular way is to employ glass as the outermost surface and bond the glass to the solar cell element with a sealant resin. Since glass is excellent in weathering resistance and is not permeable to moisture, it can be said to be one of the most excellent members for covering the photovoltaic element of a semiconductor. Therefore, most solar cell modules employ glass as the cover of the outermost surface. The glass cover, however, has problems: 1) it is heavy; 2) it cannot be bent; 3) it is weak against impact; and 4) it is expensive. These problems weigh against making use of the advantages of thin film solar cells, i.e., being lightweight, impact-resistant, and flexible.
Proposals thus have been made heretofore for lightweight, flexible solar cell modules taking advantage of the features of thin film solar cells by using a surface covering member in which the outermost surface is of a transparent fluoride polymer thin film such as a fluororesin film and in which a sealant selected from a variety of thermoplastic, transparent, organic resins is provided inside thereof. Reasons why these materials have been used are: 1) that the fluoride polymers are high in weathering resistance and water repellency, which can lessen the drop of conversion efficiency of a solar cell module due to decrease in optical transmittance resulting from yellowing or clouding caused by resin deterioration or from surface pollution and 2) that the thermoplastic, transparent resins are cheap and can be used in large quantity as a sealant for protecting the internal photovoltaic element. Normally, provided on the solar cell elements are various collector electrodes for efficiently drawing power generated and metal members for connecting the elements in series or in parallel with each other. The thermoplastic, transparent, organic resins also have an effect of smoothing the surface of a covering member by also sealing packaging members including the electrodes and metal members to fill unevenness on the surface of the element.
However, such modules wherein the surface is covered by a film, have a problem that the element is easier to scratch, different from those wherein the surface is covered by glass. Namely, when the surface is scratched by a sharp edge, even the element would be damaged readily.
In order to relieve this problem, even a little, a reinforcing material is put into the sealant resin, and nonwoven fabric of glass fiber is suitably used for this purpose. The nonwoven fabric of glass fiber also functions to maintain the thickness of the sealant resin, because the molten sealant resin is impregnated thereinto in a module lamination step by a heating press method under vacuum, and it functions to reduce bubbles remaining in the sealant resin by securing ways of escape of air when interposed between the sealant resin and the surface member and/or between the sealant resin and the solar cell element upon pressing under vacuum.
It is required that the solar cell modules are designed to endure 20-year outdoor use. Especially, high durability is required of the surface covering member comprised of the surface sealant resin layer, the surface protecting film, etc., exposed to direct sunlight. Outdoor use factors which cause deterioration include ultraviolet rays, heat, water, and so on. Among them, yellowing of the surface sealant resin layer due to ultraviolet rays and heat is a serious problem, because it results in decreasing the quantity of light reaching the photovoltaic element and thus lowering the output of the solar cell module.
Many reports have been presented heretofore about the yellowing of the surface sealant resin layer. For example, the report by SPRINGBORN LABORATORIES INC., "SEMI-ANNUAL TECHNICAL PROGRESS REPORT ON PHOTOVOLTAIC MANUFACTURING TECHNOLOGY (PV Mat), Nov. 5, 1993," describes that yellowing is likely to occur in EVA used as a surface sealant resin layer in modules installed in a desert area, e.g., in Phoenix, U.S.A. Not only in such use in the desert area, but also in use where the solar cell modules per se function as a roof, for example, like a combination roofing member and solar cell, the temperatures of modules increase up to 70.degree. C. or more and the yellowing becomes a serious problem.
Therefore, the durability of the surface sealant resin layer is enhanced by adding an ultraviolet absorbing agent, a light stabilizer, a thermal oxidation inhibiter, or the like thereto so as to endure even under severe use circumstances of the ordinary solar cell. Nevertheless, deterioration thereof unavoidably occurs because of exposure to harsh ultraviolet rays and heat over several tens of years. Thus, reliability is not sufficient yet and still more countermeasures are desired against the yellowing.
Further, the nonwoven fabric of glass fiber is often put in the surface sealant resin layer as described above, but studies by the inventor showed that it tended to be a big factor in causing the yellowing. The reason is considered to be in a binder resin for binding glass fibers with each other. As compared with the weathering resistance of the surface sealant resin layer, the weathering resistance of the binder resin in the nonwoven fabric of glass fiber is not so high per se, and the binder resin does not contain an additive for enhancing the weathering resistance. Therefore, the binder resin is more likely to be deteriorated than the surface sealant resin layer. Since the binder resin is of a different kind from the surface sealant resin layer, the binder resin is not mixed well with the surface sealant resin layer, and water thus could enter interfaces between them, promoting further deterioration.
For example, it was shown that in an example wherein EVA was used for the surface sealant resin layer and a polyvinyl alcohol resin for the binder resin in the nonwoven fabric of glass fiber, yellowing occurred mainly due to the deterioration of the binder resin in accelerated weathering tests such as the EMMAQUA test, sunshine weatherometer test, and strong ultraviolet irradiation test and in thermal degrading tests at 150.degree. C. The reason is generation of conjugated double bonds in main chains of polyvinyl alcohol polymer due to thermal dehydration.
On the other hand, a rigid resin thin film layer containing the main component of an acrylic resin is sometimes provided between the photovoltaic element and the surface sealant resin layer for the purposes of enhancing scratch resistance of the surface covering member and providing the photovoltaic element with a humidity barrier property. In this case, however, our experiment results showed that a crosslinking agent (organic peroxide) remaining in the surface sealant resin layer interacted with the acrylic resin to accelerate the yellowing of the surface sealant resin.